Logs are peeled to produce a thin continuous strip of veneer. The veneer is very thin, being on the order of 0.1 inches thick. The continuous ribbon of veneer is clipped into designated sizes to form veneer sheets. Typically the veneer is clipped into half sheet or full sheet sizes. The veneer sheets will subsequently be laminated together to form plywood.
The veneer sheets are graded according to size, quality and moisture content and are uniformly stacked for subsequent processing. Typically the veneer sheets are stacked as they come off the clipper, are unstacked for drying and are graded again after the drying cycle and restacked in individual stacks.
A veneer stacker that performs the stacking operation, whether it be green undried veneer sheets or the dried veneer sheets is of the type as disclosed in U.S. Pat. No. 4,905,843. The stacker of the '843 patent is arranged to receive veneer sheets from an infeed conveyor and accurately align each veneer sheet on its overhead conveyor. Alignment during conveyance is important for achieving alignment in the stacks where sheets that are misaligned are subject to damage.
The overhead conveyor has two pair of belts that traverse a plenum. The pairs are spaced apart to engage the veneer sheet near its side edges to transport the sheets along the length of the conveyor. The veneer stacker relies on differential air pressure to adhere the veneer sheets to the belts of the overhead conveyor. Apertures in the plenum are provided between the belts of each belt pair and when air is withdrawn from the plenum, the differential air pressure will draw the sheet toward the plenum and into frictional engagement with the belts. The stacker of the '843 patent has a controlled adjusting mechanism that will accurately adjust the position of the veneer sheet, if required, on the overhead conveyor of the stacker so that the leading edge of the veneer sheet is normal to the travel direction of the overhead conveyor. Knock off shoes are provided to accurately discharge the veneer sheets into selected bins or compartments according to size, quality, moisture content and so forth.
One of the problems is that the veneer sheets may become discriminately skewed on the overhead conveyor as it is transported along the length of the overhead conveyor even though the veneer sheets are initially accurately aligned by the adjusting mechanism. This is due in part to normal wear creating variances between the drive mechanism and the belts, i.e., the belts of a pair of belts are driven at slightly different speeds, and because of variances in the surface of the veneer sheets providing different frictional gripping properties in areas of engagement by the belts.
The drive mechanisms that drive the belts are matched and the belts are matched for uniformity of height (thickness), width and length and are selected to have the same coefficient of friction. Normal wear however causes a variance in the belts. One belt of a pair of belts may wear more rapidly than the other causing a variance in thickness.
As a belt wears it may become thinner (decrease in thickness) than the other belt. The thinner belt as it is propelled by the drive mechanism will have a slightly different velocity than a thicker belt. As a belt traverses a drive mechanism such as a circular drive wheel it is subject to a compressive and stretching action about a plane that is near the center of the belts thickness. The material of the belt nearest the drive wheel and below the plane is compressed and the material furthest from the drive wheel and above the plane is stretched. The plane about which the belt is compressed/stretched depends on the thickness of the belt. The plane in effect determines the velocity at which the belt will traverse the drive wheel. Thus a belt that is worn thin will have a different velocity than a thicker belt and any variance in the wear rate as between the belts will affect the belt velocity even though the drive wheels are driven at the same rate of rotation.
The surface of a veneer sheet can vary as between smoothness and roughness and a belt gripping the rougher surface will dominate. As between each belt in the pair of belts, one sheet may have a greater adherence to one of the belts of the pair and a second sheet will have a greater adherence to the other of the belts. The difference in velocity of the belts will accordingly cause a slight skewing of the veneer sheets as the sheets are propelled by the overhead conveyor and the skewing will be different from sheet to sheet. The sheets deposited in the stack accordingly become misaligned.